A liquid crystal display panel has a spherical spacer or a columnar spacer between a TFT (Thin Film Transistor) substrate and a CF (Color Filter) substrate. The liquid crystal display panel has a structure in which these substrates are bonded together while sandwiching the spacer between the substrates. A predetermined cell gap between the substrates is defined by the spacer.
The spherical spacer moves in a space between the substrates easily. In order to prevent the movement, a fixing layer may be provided on a surface of the spherical spacer. The fixing layer of the spherical spacer usually has only a fixing strength with which the movement and the separation of the spherical spacer caused by a turn-over of the substrate or the like can be avoided. The spherical spacer easily moves by a surface contact when performing a rubbing process or the like. For this reason, in the liquid crystal display device in which the cell gap is formed by using a spherical spacer material, the spherical spacer easily moves by vibration during transportation or the like, because the fixing strength of the spherical spacer is low. In this liquid crystal display device, a display defect such as a light leak, a reduction in contrast, a cell gap unevenness or the like is generated by the movement of the spherical spacer. An abnormal alignment occurs around the spherical spacer arranged in a display area. This causes a reduction in contrast in the display area.
Moreover, in a liquid crystal display panel, in which a high-speed response so as to handle a moving picture or the like, the high-speed response is realized by, for example, a small cell gap. However, in order to make the cell gap small, a particle diameter of the spherical spacer has to be small. Production of such a spherical spacer, with small particle diameter and small variation in particle size is quite difficult.
In contrast, the columnar spacer is fixed to one of the substrates unlike the spherical spacer. In a liquid crystal display device in which the columnar spacer is arranged in a light shielding portion, the columnar spacer is firmly fixed to one of the substrates, such as a CF substrate or the like. Therefore, a problem such as the movement of the spacer caused by vibration or the like does not occur. Moreover, because the columnar spacer is arranged in the light shielding portion, a contrast of the liquid crystal display panel is improved. The columnar spacer is smaller than the spherical spacer and a height of the columnar spacer has a small variation.
In recent years, according to demand of a liquid crystal display device having a high contrast ratio and a high speed response increases, a liquid crystal display device, in which the columnar spacer is arranged in the light shielding portion is largely produced.
The columnar spacer is fixed to one of the substrates unlike the spherical spacer. When an external stress, by rubbing a panel surface or the like is applied, the columnar spacer fixed to one of the substrates slides on the surface of the other opposed substrate. For this reason, a residual stress is generated in a substrate material by a frictional force between a top of the columnar spacer and a columnar spacer facing portion of the other substrate.
The cell gap is generally formed in a state in which the columnar spacer is compressed by several percents. A force is applied between the surface of the columnar spacer and the surface of the substrate which always contacts with the columnar spacer. Therefore, even if the force applied from outside is released, the state does not easily return to the original state because of a friction between the columnar spacer and the surface of the substrate, which contacts with the columnar spacer.
A material having small retardation, such as a glass, is generally used as a transparent substrate of a liquid crystal display device. In the panel in the above-mentioned state, the retardation is generated by the residual stress generated on a glass substrate that is caused by the friction between the columnar spacer and the glass substrate mentioned above. In an area where the retardation is generated, a light leakage occurs in a black-color display screen. When black color luminance is increased by the light leakage, the contrast is decreased and uniformity of black image quality is decreased.
When the panel is fixed by pushing an outer periphery of the panel with a module chassis or the panel is directly fixed to a module member with a double-stick tape, a stress is locally applied on the panel if the panel or the module member itself has deformation such as distortion, warpage, or the like. In such state, the retardation is generated by the residual stress of the glass substrate and the light leakage defect is generated as in the above-mentioned case.
A relation between the retardation generated by the residual stress of the glass substrate and the light leakage mentioned above will be described with reference to FIGS. 23A and 23B.
FIG. 23A shows an actual distribution of the light leakage in the panel when a load is applied to a central portion of the panel using an IPS (In Plane Switching) method. FIG. 23B shows the distribution of the light leakage predicted based on the measurement result of a magnitude and a direction of the residual stress of the panel using the IPS method.
An intensity of a light emitted by the retardation of a glass is proportional to sin(2θ), where θ is an angle between a direction of an absorption axis of a polarizer and a direction in which the residual stress is generated. Namely, as shown in the following equation, a predicted value of emission light intensity is proportional to a product of the magnitude of the residual stress and sin(2θ).I=Aτ sin(2θ)  [equation 1],
where I is the emission light intensity, τ is the magnitude of the residual stress, θ is the angle between the direction of the absorption axis of the polarizer and the direction of the residual stress, and A is a constant irrelevant to τ or θ.
A good match between the predicted value shown in FIG. 23B and the actually measured value shown in FIG. 23A is obtained. In the both distributions, as the angle between the direction of the residual stress and the direction of the absorption axis of a CF substrate side polarizer approaches 45 degrees, the light leakage becomes maximum. It can be seen that the retardation generated on the glass substrate causes the light leakage defect in the black-color display screen.
In a portion which is contact with the columnar spacer, the same phenomenon as mentioned above occurs. The direction of the residual stress generated on the glass substrate relates to the light leakage in the black-color display screen.
A liquid crystal display device described in Japanese Patent Application Laid-Open No. 2001-117103 comprises the columnar spacer. FIG. 26 shows a structure of the liquid crystal display device disclosed by Japanese Patent Application Laid-Open No. 2001-117103. In this liquid crystal display device, a columnar spacer 11 is fixed on a CF substrate 13 so as to face a TFT substrate 16. The area of the columnar spacer 11 occupies 0.05 to 0.15 percent of a pixel area of a display area. According to this area ratio, uniformity of the cell gap between the TFT substrate 16 and the CF substrate 13 is maintained, and nonuniformity of gravity, in which a liquid crystal collects in a lower portion as a temperature of a liquid crystal display panel rises, is suppressed.
In a liquid crystal display device described in Japanese Patent Application Laid-Open No. 2005-242297, a convex step is formed at a position corresponding to the columnar spacer. FIG. 27 shows a structure of a liquid crystal display device disclosed by Japanese Patent Application Laid-Open No. 2005-242297. A step 21 reduces a frictional force of a columnar spacer 20a and suppresses occurrence of display failure caused by temperature rise of the liquid crystal display panel.
In a liquid crystal display device described in Japanese Patent Application Laid-Open No. 2000-267111, the columnar spacer is formed so that the columnar spacers are faced each other. FIG. 28 shows a structure of the liquid crystal display device disclosed by Japanese Patent Application Laid-Open No. 2000-267111. The columnar spacer 33 fixed on one substrate 31 has an inclined surface which matches the inclined surface of a columnar spacer 34 that is fixed at a position opposed to the columnar spacer 33 on an counter substrate 32. When a volume of a liquid crystal is decreased with temperature decrease, a distance between two substrates varies according to a slide between the columnar spacers. As a result, an occurrence of a vacuum bubble is suppressed and a display failure is prevented.